MMUSIC Muthu A M. Perumal
Internet-Draft Cisco Systems
Updates: 4856 (if approved) Parthasarathi. Ravindran
Intended status: Standards Track Nokia Siemens Networks
Expires: December 5, 2013 June 3, 2013
Offer/Answer Considerations for G.723 Annex A and G.729 Annex Bdraft-ietf-mmusic-sdp-g723-g729-02
Abstract
RFC4856 describes the annexa parameter for G723 and the annexb
parameter for G729, G729D and G729E. However, the specification does
not describe the offerer and answerer behavior when the value of the
annexa or annexb parameter does not match in the SDP offer and
answer. This document provides the offer/answer considerations for
these parameters and updates RFC4856.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 5, 2013.
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Internet-Draft Offer/Answer G723 AnnexA & G729 AnnexB June 20131. Introduction
[RFC4856] describes the annexa parameter for G723 as follows:
annexa: indicates that Annex A, voice activity detection, is used
or preferred. Permissible values are "yes" and "no" (without the
quotes); "yes" is implied if this parameter is omitted.
Also, [RFC4856] describes the annexb parameter for G729, G729D and
G729E as follows:
annexb: indicates that Annex B, voice activity detection, is used
or preferred. Permissible values are "yes" and "no" (without the
quotes); "yes" is implied if this parameter is omitted.
However, it does not have any normative statement for the case where
the value of this parameter does not match in the SDP offer and
answer. For example, if the offer has G729 with annexb=yes and the
answer has G729 with annexb=no, it can be interpreted in two
different ways:
o The offerer and answerer proceed as if G729 is negotiated with
annexb=yes, or
o The offerer and answerer proceed as if G729 is negotiated with
annexb=no.
Since [RFC4856] does not state it clearly, various implementations
have interpreted the offer/answer in their own ways, resulting in a
different codec being chosen to call failure, when the parameter
value does not match in the offer and answer.
[RFC3264] requires SDP extensions that define new fmtp parameters to
specify their proper interpretation in offer/answer. But, [RFC4856]
does not specify it for the Annex A flavor of G.723 and the Annex B
flavors of G.729, G.729D and G.729E.
This document describes the offer/answer considerations for these
parameters and provides the necessary clarifications.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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Internet-Draft Offer/Answer G723 AnnexA & G729 AnnexB June 20133. Offer/Answer Considerations
[RFC3551] states that
Receivers MUST accept comfort noise frames if restriction of their
use has not been signaled. The MIME registration for G729 in RFC3555 specifies a parameter that MAY be used with MIME or SDP to
restrict the use of comfort noise frames.
Based on the above, it is best not to use comfort noise frames if the
SDP offer or answer indicates that comfort noise is not supported.
3.1. Offer/Answer Considerations for G723 Annex A
When the offer or answer has G723 and the annexa parameter is absent,
the offerer or answerer knows that it has implied the default
"annexa=yes". This is because the annexa attribute is part of the
original registration of audio/G723 [RFC4856]. All implementations
that support G723 understand the annexa attribute. Hence, this case
MUST be considered as if the offer or answer has G723 with
annexa=yes. .
When the offer has G723 with annexa=yes and the answer has G723 with
annexa=no, the offerer and answerer MUST proceed as if G723 is
negotiated with annexa=no.
When the offer has G723 with annexa=no then the answer MUST NOT have
annexa=yes for G723. Thus the annexa parameter can be turned off by
the answerer, but cannot be turned on.
When the offer has G723 with annexa=no, the reason for not mandating
that the answer MUST have annexa=no for G723 is that there are there
implementations that omit the annexa parameter in answer and expect
the least common denominator to be used.
When the offer has G723 with no annexa parameter and the answer has
G723 with annexa=yes, the offerer and answerer MUST proceed as if
G723 is negotiated with annexa=yes.
3.2. Offer/Answer Considerations for G.729 Annex B, G.729D Annex B and G.729E Annex B
In this section G729 represents any of G729 or G729D or G729E.
When the offer or answer has G729 and the annexb parameter is absent,
the offerer or answerer knows that it has implied the default
"annexb=yes". This is because the annexb attribute is part of the
original registration of audio/G729 [RFC4856]. All implementations
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Internet-Draft Offer/Answer G723 AnnexA & G729 AnnexB June 2013
that support G729 understand the annexb attribute. Hence, this case
MUST be considered as if the offer or answer has G729 with
annexb=yes. .
When the offer or answer has G729 and the annexb parameter is absent,
it MUST be considered as if the offer or answer has G729 with
annexb=yes.
When the offer has G729 with annexb=yes and the answer has G729 with
annexb=no, the offerer and answerer MUST proceed as if G729 is
negotiated with annexb=no.
When the offer has G729 with annexb=no then the answer MUST NOT have
annexb=yes for G729. Thus the annexb parameter can be turned off by
the answerer, but cannot be turned on.
When the offer has G729 with annexa=no, the reason for not mandating
that the answer MUST have annexa=no for G729 is that there are there
implementations that omit the annexa parameter in answer and expect
the least common denominator to be used.
When the offer has G.729 with no annexb parameter and the answer has
G.729 with annexb=yes, the offerer and answerer MUST proceed as if
G.729 is negotiated with annexb=yes.
4. Examples4.1. Offer with G729 annexb=yes and answer with G729 annexb=no
[Offer with G729 annexb=yes]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 18
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=yes
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